Efficiency and emission numbers: how much difference is significant?

[poltroon]

I am looking at new woodstoves. Currently, I am working with a Heartland kitchen cookstove as our main source of heat, so I'm pretty comfortable with using wood heat.

I am comparing stoves from Hearthstone, Jøtul, Vermont Castings, and Quadra-Fire, and looking at the efficiency and the emissions numbers. Since my DH has asthma, minimizing emissions is fairly important to us.

Looking at the numbers, the emissions reported range from 0.7 to 3.2 grams/hr. I am certain this is substantially better than the current stove. But how good is it? Will I notice a difference between 3.2 and 0.7? What about say 3.2 and 1.5? Can I expect these numbers to stay stable over the life of the stove (these are all non-catalytic).

Obviously less is better, and I'd prefer to add as little to the atmosphere as possible.

Similarly, efficiency numbers range from 81% to " > 75%" to 68% (although still an EPA qualified stove) to 77%. It seems to me I might not notice 5% but at 10% I might notice. Or then again, perhaps these numbers aren't so consistent that over time, when I'm using it, that I'll notice. (Certainly it will be better than what I have.)

It surprises me that the stove with the lowest reported thermal efficiency also reports the lowest emissions per hour. That is counterintuitive.

Any thoughts?

 

[poltroon]

On efficiency, I'm thinking that the 5% difference is roughly 1 log in 20, and the 10% is maybe 1 log in 10. The first I wouldn't notice; the second I would start to notice, maybe.

But here's another thing that confuses me about emissions.

Vermont Castings shows the Defiant at 60,000 BTU per hour, 69% efficiency, and .75 grams/hr.
Their Aspen is 18,000 BTU per hour, 81% efficient, and 4.3 grams per hour.

The Defiant is going through roughly 4 times as much wood per hour... and yet has 1/6th the emissions? It's 24x better in emissions per log? Seriously?

 

[jharkin]

The information on VC web site is all over the map. If you look at the defiant specs, the catalyst version is more efficiency (expected) but lists higher g/hour emissions. Also the specs on the web page summary are different than the pdf "spec sheet" for the exact same stove.

Either poor editing, or they re-tested/revised the numbers through updates to the stove and/or during ownership changes, and didn't keep the web site current.


BTW before you consider a VC non-cat, suggest you search the forums for "everburn".

 

[precaud]

There is no standard efficiency test that has been adopted industry-wide, so unfortunately you can't compare stoves based on the numbers given by the manufacturers.

The emissions numbers are a little more useful, but still less than definitive. Generally speaking, a lower EPA number does signify a cleaner-burning stove, but it doesn't necessarily identify a more efficient one. Clean burning is necessary but not sufficient by itself for high efficiency.

 

[Renovation]

There is no standard efficiency test that has been adopted industry-wide, so unfortunately you can't compare stoves based on the numbers given by the manufacturers.

The emissions numbers are a little more useful, but still less than definitive. Generally speaking, a lower EPA number does signify a cleaner-burning stove, but it doesn't necessarily identify a more efficient one. Clean burning is necessary but not sufficient by itself for high efficiency.

Agreed.

My attempt at comparing efficiency specs ended in frustration, for the same reason. Added two it, there are two testing methods, indicated by alphabet-soup acronyms something like ldhf and hdhf that give different results. One calculates the efficiency *including* the energy lost by vaporizing the moisture in the wood, and the other subtracts it out, resulting in about a 10% higher number for the same stove. Many manufacturer specs don't say which method they are quoting, so it is just a big mess.

Added to that, I believe there are EPA "estimates" that a manufacturer can quote instead of a specific number for their stove, and the cat estimate is higher than the non-cat thermal smoke burners. Yay. Good luck figuring it all out. If you do, please post and let us know.

Someone here made a guesstimate that EPA chamber thermal reburners (eg Everburn, Leyden) are about 10% more efficient than non EPA stoves, air injector secondary reburners (typical non-cat EPA) are another 10%, and cats another 10%. Those are all WAGs of course, and it will depend on the stove. I guess the best one can do is go by user reports.

 

[poltroon]

I appreciate the tip to search 'everburn.' The same dealer reps all these stoves, and they sell quite a lot of them. I had heard some negative comments about VC, but he said they haven't had a lot of problems with the units they've sold. He also says that for the most part he has a lot of repeat business within the same lines - that once someone has a Jøtul, they keep buying Jøtul, and if they start VC, they tend to come back for another VC.

The very low reported emissions for VC are very attractive - but given that they seem like the stoves are fussy, I wonder how many stoves achieve those extraordinary values 'out in the wild.'

Googling around, I found this chart comparing various pollution sources in grams per hour:
http://burningissues.org/car-www/medical_effects/comp-emmis-part-sources.htm

Source with reference # Emissions in Grams/hour or day
(30) Cigarette .4 grams/hour (0.8 grams/pack)
Gas or Propane Furnace (4) .001 grams/hour ( 0.024 grams/day)
Oil furnace .02 grams/hour (0.48 grams/day)
Pellet Stove (4) 2.4 grams/hour ( 56.6 grams/day)
Single Simulated Log (1, 1a) 8 grams/hour
Non-certified wood stove 15.6 grams/hour
Fireplace-hardwood (1,3): (36 lbs. or 16 kg burned over 3 hours.) 30 grams/hour
Fireplace-softwood (1,3): (31 lbs.or 14kg burned over 3 hours.) 59 grams/hour.
Auto-with Catalytic Converter (1,2) .66 grams/hour
Auto-without Catalytic Converter (1,2) 3.5 grams/hour.
Auto-smoking 6 grams/hour
Diesel 14 ton Truck or Bus >1994 (2) 36 grams/hour
Diesel Truck or Bus < 1993 (2) 70 grams/hour

 

[precaud]

He also says that for the most part he has a lot of repeat business within the same lines - that once someone has a Jøtul, they keep buying Jøtul, and if they start VC, they tend to come back for another VC.

I think that is more for visual/aesthetic reasons than anything else. You've already done more research than most buyers do...

 

[jharkin]

If the lowest possible emissions is your primary concern in the decision, you might want to consider including catalytic stoves in your options. Both Blaze King and Woodstock Soapstone have very loyal followings here and are very clean burning. And the VC catalytic models are known to be less temperamental than their everburn models (I own a VC cat and its not half bad).